Liners are often employed in gas turbine engines to enclose high temperature, core gas paths. An augmentor which forms an additional combustion chamber for afterburning the gas turbine exhaust, may use a cylindrical liner to define a flow path for the core gas flow from the turbine to the exhaust nozzle. The liners within the augmentor duct protect the duct from excessive temperatures associated with the turbine exhaust.
Liners typically include an outer wall, an inner wall and a means for spacing the two walls a distance apart. A duct defined by such spaced walls is used as a cooling air passage. Cooling air is supplied between the liner walls for convection cooling of the liner. Cooling air also flows inwardly through perforations in the outer wall of the liner to provide impingement cooling of the inner surface of the inner wall of the liner. Perforations in the inner wall of the liner provide a path for the cooling air to exit into the core gas flow.
It is known to provide liners with stiffening structure ("stiffeners") extending between the two walls to maintain spacing the two walls a distance apart. Fasteners attach the stiffeners and the walls together and collectively the stiffeners and the fasteners form a rigid cylindrical structure. Stiffeners protect the liners from buckling or collapsing under differential pressure loads between the outer and inner walls. For example, a significant pressure difference exists across the liner during stall conditions. During a stall condition, a higher pressure exists surrounding the outer wall of the liner relative to the pressure on the inner liner wall. This creates a tendency for the liner to collapse or buckle under the external loading. The sizing of the perforations in the inner and outer liner walls do not allow for enough air flow to form the outer to the inner liner wall to diffuse the pressure loading across the liner walls.
Further, high core gas temperatures can also limit the useful life of the augmentor liner. The inner wall of the liner may operate on the order of 500 degrees Fahrenheit hotter, than the surrounding outer wall of the liner. Cooling schemes such as described hereinabove and thermal barrier coatings are typically used to provide cooling to decrease the rate of thermal damage to the liner walls but do not solve the problem caused by the differential thermal growth of the liner walls.
The stiffeners in the liners of the prior art design provide a very stiff structure that results in low cycle fatigue problems for the liner. In addition, due to differential thermal growth and movement between the outer wall and the inner wall of the liner, the liner experiences undesirable stress. There is a high probability that the stress will result in cracks in the coating disposed on the inner wall of the liner. The stress and cracks compromise the durability of the liner.